The present invention relates generally to fluid dispensing devices and more particularly to metered-dose pump devices useful for dispensing high-viscosity semi-solid substances.
It is common in the cosmetic industry and the medical industry to provide liquids and pastes in dispensing containers. For example, topical ointments and creams are often provided in dispensing containers configured with pump mechanisms that deliver a specified amount or dose of the fluid in response to user-actuation of the pump mechanism. Examples of such containers and pump mechanisms are shown in U.S. Pat. Nos. 5,217,050, 5,950,880 and 5,961,005, which are each hereby incorporated by reference in its entirety. Typical dispensing devices as shown therein include a substantially tubular hollow container body having a substantially circular cross section fitted with a finger-operable pump mechanism and a follower, or take-up, piston. The finger-operable pump includes a dip tube having an inlet at its end that provides the fluid to a pump chamber for discharge at a dispensing outlet. As fluid product is dispensed, the follower piston slides toward the pump mechanism due to atmospheric pressure acting on the exterior surface of the piston. This action decreases the internal volume of the body by an amount equal to the volume of product discharged. The piston may be fully enclosed by the container body, in which case a vent is typically provided to ensure atmospheric pressure at the external surface of the piston.
FIGS. 1 and 2 illustrate a dispenser configured with a typical follower piston 25 having a well 40 adapted to mate with the receiving end of a dispensing valve, or dip tube 35, when the piston 25 reaches its uppermost position (FIG. 2, associated with a reduced content of product in the body). In general, such a configuration works well for fluid or semi-solid preparations with very low viscosity. However, when more viscous fluids and semi-solid preparations are used, the accuracy of the volume extracted through the inlet into the pump chamber and ultimately dispensed is compromised due to restricted flow of the viscous medium into the piston well around the dip tube. In the particular case of highly viscous material (e.g., greater than about 2,000 cps) the pump mechanism may quit pumping or it may deliver inaccurate volume after a significant portion (e.g., about 70%) of the material has been depleted. In this situation, the gap between the pumping tube and the walls of the piston well is generally too limited to allow slightly viscous to highly viscous material to flow freely into the pumping well. Thus, without sufficient material flowing into the well during actuation of the pump mechanism, the volume delivered may be inaccurate and in some cases the pumping mechanism may not work.
It is therefore desirable to provide dispensing devices that overcome the problems associated with prior devices, and which efficiently dispense fluid, particularly highly viscous substances, after a significant portion of the fluid has been dispensed.